1. Field of the Invention
This invention relates to a brain function measuring apparatus which uses nuclear magnetic resonance to discriminate and measure differences between the functions of the left and right hemispheres of a brain in vivo, the state of the functioning and the active portion locations.
2. Description of the Prior Art
There is known the technique of nuclear magnetic resonance (NMR) for recognizing fine structural changes occurring in the tissues of an organism, by imparting an oscillating magnetic field (resonant frequency) which is generated by, for example, high frequency to the spins of atomic nuclei, of which the organism is composed so as to incline a microscopic magnetic moment of the spins and detecting a free induction signal produced when the energy level of the nuclei represented by the microscopic magnetic moment reverts to the ground state. The theoretical technology has been established for using NMR to recognize in vivo blood conditions such as blood flow amount, blood flow velocity, amount of residual blood, and so forth.
It is known that when an external stimulus is applied to the living brain or when the brain cerebrates mentally, fine changes occur in the aforementioned type of blood states at the parts of the brain where such activities occur. It is considered that, in the utilization of NMR, brain activity will produce fluctuations in the order of 0.01 to 0.1 ppm in the intensity of a signal derived from the resonance of the protons of the active portion of the brain that is obtained as a free induction signal. If these minute changes can be detected, brain activities can be recognized. (See NMR Igaku, Vol. 6 March 1986 Proceedings of 7th Conference of Japanese Society of Magnetic Resonance in Medicine.)
In the determination of brain functions using the above type of NMR technique, when the resonance signals from protons in water or loosely-bound fats at the site of the activity are observed, the resonance signals undergo minute changes that are in accordance with the blood flow amount, the blood flow velocity and the residual blood amount. However, the obtained resonance signals are not only from protons in the blood flow, but are instead from protons in all the tissues, and the intensity fluctuation in resonance signals coming from the blood flow accompanying brain activity will account for no more than about 0.01 to 0.1 ppm of the resonance signals from the whole of the tissues.
With such resonance signal observation techniques, various signals can be obtained from NMR. Changes in blood flow velocity has an effect on the spin-echo signal in the state when a magnetic gradient has been applied, and a change in the residual amount of blood produces a change in the intensity of the free induction decay signal.
Therefore, if the free induction decay signals or spin-echo signals are observed, changes in the blood flow can be found from the changes in the said signals, and the comparative identification of these changes arising from brain activity can enable brain activity to be recognized.
With the conventional apparatus for determining brain function using the phenomenon of NMR, a problem has been that the computing of the minute changes in the proton resonance signal involved the use of such computation processing as accumulation and Fourier transforms, and to implement this, the resonance signal obtained is quantized by an A/D converter. However, because the said change is in the order of a minute 0.01 to 0.1 ppm, the detection signal was buried within a single one of the bits that form the quantization unit, so the change cannot be detected from the digital output.